1. Field of the Invention
Aspects of the present invention relate to a direct liquid feed fuel cell, and more particularly, to a direct liquid feed fuel cell having a structure that prevents an oxygen supply path from being blocked by water produced at a cathode electrode.
2. Description of the Related Art
A direct liquid feed fuel cell is an apparatus that generates electricity through an electrochemical reaction between an organic fuel (such as methanol or ethanol) and an oxidant (such as oxygen). A direct liquid feed fuel cell has high specific energy density and current density. Also, since a liquid fuel (such as methanol) is fed directly to the cell, the direct feed fuel cell does not require a peripheral device, such as a fuel reformer, and the storage and supply of the liquid fuel is easy.
As depicted in FIG. 1, a unit cell of a direct feed fuel cell has a membrane electrode assembly (MEA) structure including an electrolyte membrane 1 interposed between an anode electrode 2 and a cathode electrode 3. The anode electrode 2 includes a diffusion layer 22 for supplying and diffusing fuel, a catalyst layer 21 at which oxidation reaction of the fuel occur, and an electrode supporting layer 23. The cathode electrode 3 includes a diffusion layer 32 for supplying and diffusing an oxidant, a catalyst layer 31 on which reduction reaction of the fuel occurs, and an electrode supporting layer 33. Conductive plates 4 and 5 are respectively installed on the electrode supporting layers 23 and 33. The conductive plates 4 and 5 respectively include flow channels 41 and 51.
An electrode reaction of a direct methanol fuel cell (DMFC), which is a type of direct liquid feed fuel cell, includes an anode reaction in which fuel is oxidized and a cathode reaction in which hydrogen and oxygen are reduced as described below.CH3OH+H2O→CO2+6H++6e31 (Anode reaction)  Reaction 13/2 O2+6H++6e→3H2O(Cathode reaction)  Reaction 2CH3OH+3/2 O2→2H2O+CO2 (Overall reaction)  Reaction 3
Carbon dioxide, six hydrogen ions, and six electrons are produced at the anode electrode 2 where the fuel is oxidized (reaction 1). The hydrogen ions migrate to the cathode electrode 3 through the hydrogen ion exchange membrane 1. At the cathode electrode 3, water is produced through the reduction reaction (reaction 2) between hydrogen ions, electrons transferred from an external circuit, and oxygen. Accordingly, water and carbon dioxide are produced as the result of the overall electrochemical reaction (reaction 3) between methanol and oxygen.
The theoretical voltage that can be generated by a unit cell of a DMFC is approximately 1.2 V. However, the open circuit voltage at ambient temperature and atmospheric pressure falls below 1 V due to a voltage drop caused by an activation overvoltage and an ohmic overvoltage. In reality, the actual operating voltage lies between 0.4 and 0.6 V. Therefore, to obtain higher voltages, a plurality of unit cells are connected in series. Methods of connecting the unit cells include a monopolar structure, in which a plurality of unit cells are formed on one electrolyte membrane, and a stacked structure, in which a plurality of unit cells are stacked.
Flow channels 41 are formed on a surface of the conductive plate 4 facing the cathode electrode 3. When water produced at the cathode electrode 3 forms large drops on the surface of the electrode supporting layer 33, the water drops blocks air flow, which results in unstable power generation.